1. Field of the Invention
This invention relates to preparing metal bodies by injection molding, more particularly, the invention pertains to a method for preparing a metal body having improved mechanical properties.
2. Description of the Prior Art
The metal body of the type described in this invention is manufactured by kneading a metal powder with an organic binder, injection-molding the kneaded mixture to form a green body, removing the organic binder from the green body to form a brown body, and sintering the brown body. The method described above is superior in that it allows an arbitrary selection of the shape of the metal body, that it is suitable for mass production, and that the sintered product has excellent physical and mechanical properties because of the improved compaction obtained by the use of fine powder. However, a substantial amount of organic binder must be used relative to the metal powder amount in order to give plasticity to the metal powder and prevent deformation of the green body molded into a prescribed shape. This entailed a troublesome step of removing the organic binder from the green body, i.e., debinding the green body, over a prolonged period of time at a very gradual temperature gradient.
As a known method, Japanese Unexamined Published Patent Application No. 61-204301 discloses a method wherein short fibers of synthetic resins such as, polypropylene, nylon and acrylic in the range of from 10 to 40 .mu.m in diameter and from 0.3 to 2 mm in length are kneaded with a metal powder and an organic binder. In this method, the short fibers are used to prevent deformation of the green body after molding, so that the amount of the organic binder can be reduced and debinding facilitated. Thus, this method is superior in that no cracking occurs in the brown body after debinding the green body to thereby obtain a sintered body with high quality and high strength. The short fibers used in this method are, however, of a synthetic resin, which become softened during debinding the green body with temperatures reaching as high as 350.degree. C. and eventually become melted. This method is defective because of insufficient performances in preventing deformation of the green body during debinding and in maintaining the strength of the brown body after debinding.